1. Field of the Invention
The invention relates to a process and apparatus for use on a vehicle, such as an airplane, to detect when weather conditions will cause ice to form on the vehicle.
2. Description of the Prior Art
Vehicles such as airplanes may be easily disabled by the formation of ice upon them. Accordingly, the detection of weather conditions which permit the formation of ice on a vehicle is of great practical concern. For instance, when an aircraft is in flight, the formation of ice upon it may restrict or immobilize control surfaces such as the ailerons, flaps, elevators, and rudder. Orifices such as pitot tubes, engine air intakes and so forth may also be partially blocked or completely obstructed by the formation of ice. If ice affects the movement of a control surface, the aircraft may become uncontrollable. Ice in a pitot tube may give false instrument indications to the pilot, resulting in possibly serious disorientation of the aircraft. Formation of ice in an air intake may cause severe engine damage or engine failure.
In addition, ice which forms on an aerodynamic surface may cause an adverse change in the aerodynamic contours of the surface, preventing continued flight of the aircraft due to loss of lift. Formation of ice anywhere on a plane may result in a change in its center of gravity, making the plane uncontrollable.
It has been apparent for many years that an aircraft pilot must either avoid potential ice forming weather conditions or insure that appropriate ice removal mechanisms on the airplane are enabled when ice could form on the airplane.
There are two basic requirements for weather conditions to allow the formation of ice on a vehicle. First, there must be free water vapor in the atmosphere available to form ice. That is to say, although the temperature may be -50 degrees C., if there is no free water vapor, i.e. the relative humidity is low, ice cannot form.
Second, the temperature of the vehicle must be at the freezing point of water or below. However, if the vehicle is moving, i.e. if it is an aircraft in flight, the ambient weather temperature may be below the freezing temperature of water, yet the motion of the vehicle though the air will heat the airflow above ambient temperature. If the ambient temperature is slightly below freezing, yet the vehicle is moving with sufficient velocity, ice may be prevented from forming by the effect of the vehicle upon the temperature of the air flowing past it.
With respect to an aircraft, it should be noted that the freezing point of water is also affected by the pressure. As pressure decreases, as happens when an airplane climbs, the freezing point of water will increase in temperature.
Prior art approaches to a process for detecting ice forming weather conditions have been unsatisfactory. For instance, the effect of the air velocity of the vehicle is usually not considered. This is an important consideration in the design of an accurate ice detection apparatus for use with an airplane that has operating velocities ranging over several hundred knots.
Another problem with prior art devices is the false generation of an ice present signal. Such false signal generation may occur in many ways. For instance, a sensor probe, if heated by the apparatus to remove ice on it, may have a temperature above that of the aircraft, therefore ice may not form on the probe yet form on the aircraft. Another problem is distinguishing in inclement weather whether ice or merely water is on the sensor probe.
Another problem with prior ice detection apparatus is the providing of a sensor probe which is sensitive and responsive, yet sufficiently hardy to withstand the extreme weather conditions to which it will be subjected.
A prior art ice detection apparatus is disclosed in U.S. Pat. No. 2,269,019, issued Jan. 6, 1942, entitled "Warning Device" to E. P. Hall. The Hall Patent teaches the utilization of a resistance element, whose resistance changes inversely in proportion to increasing temperature, which is exposed to the weather conditions. The resistance element is connected in series to a relay coil. A current is passed through the resistance element, and when its resistance is sufficiently low, the relay is energized to light an indicator lamp. A heating element in close proximity to the resistance element is periodically turned on to raise the temperature of the resistance element. If no ice is present on the resistance element, its temperature should rise sufficiently so as to increase its resistance to the extent that the relay will open, turning off the lamp. If icing conditions are encountered, the heating element will not heat the resistance element sufficiently to turn off the lamp. Accordingly, a steady state illumination of the lamp should indicate that ice forming conditions prevail. A blinking lamp should indicate that the resistance element is being exposed to "near" icy weather conditions.
The Hall apparatus, however, will not provide a reliable indication of ice forming weather conditions. For instance, water on the resistance element of Hall will have an effect similar to that of ice. The Hall apparatus depends upon the specific heat capacity of the ice to slow the temperature increase of the resistance element when the heater is turned on. However, water will provide a similar effect to that of ice in that its specific heat capacity may also cause the indicator lamp to remain on. Accordingly, the Hall device will generate false ice present signals when the weather conditions are inclement but not icy.
The Hall device also provides no compensation for the effect an aircraft's velocity through the air has upon the formation of ice upon it. The temperature of the resistance element at the end of the heater cycle is dependent not only on whether ice or water is on it, but also on the amount of heat bled off by the flow of air across it, a significant factor with high speed aircraft.
Another disadvantage to the Hall device is that it makes no provision to prevent the heating of the resistance element above the temperature of the aircraft. Accordingly, the Hall device may not generate an ice present signal when appropriate.
Another disadvantage to the Hall device is that it provides a blinking indicator lamp when conditions are potentially icy, the blink duration increasing whenever ice or water is on the resistance element until the lamp remains on steadily. This complicated interpretation of the indicator lamp is undesirable in modern aircraft. A simple yes/no indication is preferable.
Another disadvantage with the Hall device is that if the aircraft is flying in extremely cold dry weather, the indication lamp of the Hall device may remain on if the heater is unable to raise the temperature of the resistance element sufficiently due to the extreme cold. Nevertheless, the weather conditions would not permit the formation of ice due to the lack of free water vapor.
Accordingly, it is an object of the herein disclosed invention to provide a process and apparatus for detecting when weather conditions allow ice to form upon a moving vehicle. A further object of the invention is to provide such ice detection with compensation for the effect of the vehicle velocity upon the formation of ice upon it. It is a further object to provide a sensitive and responsive ice detection without the generation of false indications of ice forming conditions. It is a further object of the invention to provide a ice detection probe which is sensitive and responsive, yet sufficiently hardy to withstand the extreme weather conditions to which it will be exposed.